The present invention pertains to systems and methods involved in radiocommunication systems and, more particularly, to systems that employ binary signal streams of known amplitude which are disturbed by, for example, DC offsets, drifts, and other slowly changing disturbances superimposed on the desired binary signal. The techniques described herein are particularly well-suited for the detection of binary FM or binary FSK modulated signals in the presence of such disturbing signals, but can also be used in conjunction with other types of modulation.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry""s growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as to maintain high quality service and avoid rising prices.
FIG. 1 illustrates an example of a conventional cellular radio communication system 100 in which the present invention can be implemented. The radio communication system 100 includes a plurality of radio base stations 170a-n connected to a plurality of corresponding antennas 130a-n. The radio base stations 170a-n in conjunction with the antennas 130a-n communicate with a plurality of mobile terminals (e.g. terminals 120a, 120b and 120m) within a plurality of cells 110a-n. Communication from a base station to a mobile terminal is referred to as the downlink, whereas communication from a mobile terminal to the base station is referred to as the uplink.
The base stations are connected to a mobile telephone switching office (MSC) 150. Among other tasks, the MSC coordinates the activities of the base stations, such as during the handoff of a mobile terminal from one cell to another. The MSC, in turn, can be connected to a public switched telephone network 160, which services various communication devices 180a, 180b and 180c. 
In conventional cellular radiocommunication systems such as that illustrated in FIG. 1, the signal transmitted over the air interface does not travel along a single, straight path. Instead, the radiated energy reflects and travels in many directions so that different portions of the radiated energy arrive at the receiver (i.e., either that of terminals 120 or base stations 170) at different times. As a result, the receiver receives a distorted signal that is very different from the original signal. This distortion problem, which is commonly referred to as multipath fading, can be viewed as a smearing of the transmitted pulses.
In such conventional systems, the effects of the radio channel are measured and taken into account in the receiver when attempting to correctly determine the originally transmitted information. Channel estimates are calculated based upon known information which is periodically transmitted over the radio channel to the receiver. Since radio channels may change rapidly, e.g., due to movement of the terminals 120, the channel estimate can be regularly updated.
Channel estimation can be used in conjunction with an application of the Viterbi algorithm to determine the originally transmitted information as shown in FIG. 1(b). Therein, the received signal is used to produce channel estimates at block 200. The channel estimates are provided to the Viterbi detector 220, wherein they are employed to determine metrics associated with the likelihood of various state transitions. Those skilled in the art will readily understand the operation of Viterbi detector 220 and, therefore, a fuller discussion of this device is not provided here. A filter 240 may also be provided upstream of the Viterbi detector 220 to whiten the noise associated with the earlier processing (not shown) on the received signal, since it has been shown that Viterbi detectors provide optimal results in the presence of white, rather than colored, noise.
Although channel effects are a dominant disturbance in conventional cellular systems, in other types of systems the dominant disturbance to transmitted signals may arise from other sources. For example, a new relatively low cost, low range wireless transmission system (defined by the recently developed xe2x80x9cBluetoothxe2x80x9d technology) has been proposed for localized two-way data transmissions. Bluetooth systems are envisioned as a universal radio interface in the 2.45 GHz frequency band that enable portable electronic devices to connect and communicate wirelessly via short-range, ad hoc networks. Readers interested in various details regarding the Bluetooth technology are referred to the article entitled xe2x80x9cBLUETOOTHxe2x80x94The universal radio interface for ad hoc, wireless connectivityxe2x80x9d authored by Jaap Haartsen and found in the Ericsson Review, Telecommunications Technology Journal No. 3, 1998, the disclosure of which is incorporated here by reference. Of particular interest for this discussion is the fact that channel effects associated with the Bluetooth air interface may not be the dominant disturbance to transmitted signals in such systems, due to the short-range nature of the air interface links. Accordingly, other slowly varying disturbances may be more problematic than channel effects in such systems.
Such disturbances can have several origins. In many instances, the disturbance cannot be filtered out when the desired signal itself has low-frequency components. Examples of such disturbances include DC offset in homodyne receivers, offset in FM discriminators due to inaccuracies in the local oscillator frequency, drift (in otherwise presumably constant signal levels) due to temperature and aging, all of which represent situations where special attention has to be given to obtain error-free recovery of the desired signal.
There are several methods for performing DC offset suppression. The simplest methods use DC blocking capacitors to high-pass filter the signal. However, these filters have long response times which result in long settling times after turning on the receiver. Such long settling times are unacceptable in TDMA receivers where the receiver is switched on and off repetitively. Another technique for performing DC offset suppression is differentiation followed by integration. The differentiation removes all DC components since it has a zero at DC. Integration inverse filters the differentiated signal. The differentiation and integration can conveniently be carried out using adaptive delta modulation (ADM) techniques, e.g., as described in U.S. patent application Ser. No. 07/578,251, entitled xe2x80x9cDC Offset Compensationxe2x80x9d, filed in September of 1990 to Paul W. Dent. However, this technique requires considerable oversampling and can only be used to suppress DC offset. Drifts and other slowly varying, unwanted signals cannot be suppressed. Other suppression techniques are carried out in the digital domain, but require a high dynamic range of the A-to-D converter since no suppression has taken place prior to the digital processing.
Accordingly, it would be desirable to provide a solution to address the problems associated with slowly varying disturbances, such as DC offset, drift, etc.
These, and other, drawbacks, limitations and problems associated with conventional techniques for compensating for slowly varying disturbances are overcome by the present invention which removes slowly varying disturbances superimposed on streams of binary symbols with known separation. According to one exemplary method, the signal is first sampled at the symbol rate. The symbols are then fed into a discrete, finite-impulse-response (FIR) filter that removes the disturbance. The effects of the filter on the desired binary signal (e.g., intersymbol interference) are undone in a decoder that applies a Viterbi algorithm and acts like an equalizer. The Viterbi algorithm uses the knowledge of the constant amplitude separation of the binary signals, and forms estimates of the filter response for different possible desired input sequences.
The complexity of the exemplary systems according to the present invention depends on the length of the FIR filter. The longer the FIR filter, the more states are required in the Viterbi algorithm. The performance of the system depends on the coefficients of the FIR filter. Exemplary embodiments of the present invention describe how to tailor the trade-off that can be made between the noise performance and the suppression performance of the FIR filter.